Screening

Question 1

What is Screening?

Answer 1

The use of testing to sort out apparently well persons (asymptomatic) who probably have disease from those who probably do not

Question 2

Diagnostic test

Answer 2

used for persons in whom disease is suspected, due to symptoms, physical findings, or laboratory findings.

Question 3

Screening tests

Answer 3

used for identifying early signs of disease in people with no symptoms of the disease.

Question 4

Screening increases

Answer 4

Lead time

Question 5

Lead time

Answer 5

Interval by which the time of diagnosis is advanced by screening and early detection compared to usual time of diagnosis

Question 6

How good is the screening test?

Answer 6

We want the test to accurately and reliably measure the truth

Valid and Precise

Question 7

How do we know true disease status?

Answer 7

The Gold Standard is the best test available; it is the definitive diagnostic test

Screening tests are compared against the Gold Standard

Question 8

Validity

Answer 8

Ability of a test to distinguish between who has disease and who does not

Valid tests have both high sensitivity and high specificity

Question 9

Sensitivity

Answer 9

Ability of a test to correctly identify those who have the disease

Question 10

Specificity

Answer 10

Ability of a test to correctly identify those who do not have the disease

Question 11

Screening Program Evaluation Pro

Answer 11

We could observe benefit and early detection IF:

1. All or most clinical cases of the disease go through a detectable preclinical phase
2. In the absence of intervention, all or most cases in a preclinical phase progress to a clinical phase

Question 12

Screening Program Evaluation BUT

Answer 12

However, usually:
•Some cases progress too rapidly through a detectable preclinical phase to actually be detected by screening
•Some cases of preclinical disease never progress to clinical disease (remain preclinical or spontaneously regress)

Question 13

Why we don’t screen for everything

Answer 13

• Cost
• Possibility of doing harm – pain/discomfort
• Complications
• If no good treatment is available
• Effects on insurability
• Wrong answers create:
• Anxiety (false positives)
• Additional tests (false positives)
• Unnecessary treatment (false positives)
• False sense of security (false negatives)

Question 14

Criterion for screening programs

Answer 14

Importance/seriousness of outcome
Alter natural history of disease
Prevalence of disease in target population
Technically feasible, consider costs & benefits
Acceptable to individual and healthcare professional
Recognized treatment

Question 15

Sensitivity calculation

Answer 15

diseased who screen positive / all diseased

a / a+c

Question 16

Specificity calculation

Answer 16

non-diseased who screen negative / all non-diseased

d / b+d

Question 17

Positive Predictive Value

Answer 17

If test (+), the probability is that patient has disease

Question 18

Negative Predictive Value

Answer 18

If test (-), the probability is that patient does NOT have disease.

Question 19

PPV and NPV are both affected by:

Answer 19

Prevalence of disease increases: PPV increases, NPV decreases
Specificity/Sensitivity of test: specificity/sensitivity increases, PPV increases

Question 20

Calculation of PPV

Answer 20

true positives / all tested positives

a/a+b

Question 21

Calculation of NPV

Answer 21

true negatives/ all tested negatives

d/ c+d